We are applying several different approaches to the elucidation of the preferred backbone conformations of the hypothalamic releasing factors, LRF and TRF. We have synthesized analogs of LRF which are specifically substituted with the nicotinamidium group at the 8-position in order to examine the intramolecular charge-transfer interaction between the nicotinamidium group and the tryptophan residue at the 3-position. Our second approach has been to examine the central tetrapeptide sequence (Ser-Tyr-Gly-Leu) of LRF and analogs, which are thought to adopt a beta-turn in the LRF molecule. Thirdly, we are attempting to synthesize cyclic LRF molecules which "lock" the molecule in the proposed beta-turn, and would therefore provide analogs with enhanced biological activity if this conformation is important at the receptor site. Finally, we have synthesized analogs of TRF and LRF which contain p-amino-D-phenylalanine. These analogs have high binding constants and potencies and the p-NH2Phe residue also offers the possibility of derivatization for affinity labelling and photoaffinity labelling of the receptors for TRF and LRF and of purification of their receptor proteins via affinity chromatography. BIBLIOGRAPHIC REFERENCES: B. Donzel, M. Goodman, Rivier, N. Ling, and W. Vale. Synthesis and Conformations of Hypothalamic Hormone Releasing Factors: (N alpha MeHis2)TRF and (N alpha MeAla3)TRF, Two Biologically Active Analogs. Nature, 256, 750 (1975). C. Sakarellos, B. Donzel and M. Goodman. (4-Amino-D-Phenylalanine6)Luliberin. A Biologically Active Analog of the Luteinizing Hormone-Releasing Factor Suitable for Affinity Labeling. Biopolymers (1976), in press.